Flight Instructor on training flight with student reported hostile and confusing ATC communications.

Narrative

This was a training flight with an instrument student. We were VFR going to the northwest practice area; 15 miles northwest of VGT. On the initial departure climb; ATC told us to maintain 3;000 ft. We complied. Upon exiting the Delta airspace I initiated a climb for safety. I was 400 ft. AGL over a heavily populated area with rising terrain in front of me. Upon initiating a climb after clearing the airspace; I saw traffic to the east at my 9 o'clock flying directly at us and approximately 1 mile and 500 ft. above us. We were clear of the Delta airspace and the traffic was no factor. Upon initiating my climb ATC controller told us 'I told you to maintain 3;000 ft. That was because I have other traffic entering the airspace above you. Now you can climb; frequency change approved.' The tone was hostile and sarcastic which was highly distracting while being so low to the ground and with a student next to me. No additional traffic advisories were issued upon exiting the airspace and we were not notified to maintain frequency or altitude upon exiting the Delta. If the expectation was to maintain altitude upon exiting the airspace this should have been communicated to us by the controller. Additional traffic advisories should have been communicated. I should not have been told to maintain 3;000 ft.; that altitude was too low. The normal procedure is to exit the airspace at 4;000ft. Additional traffic should not have been routed directly over us outside of the airspace. For safety; I always monitor the tower until 10 miles from the airport. VGT Tower does not always issue 'frequency change approved' which caused confusion when the controller told us we were cleared to climb while already outside of the airspace. I attempted to call the tower to speak with the controller to solve the issue. The controller was hostile; continuously spoke over me then set the phone down and walked away. I called the supervisor and left a voicemail in an attempt to solve this issue.